Interesting. I thought it was because the river emptied into the Gulf closest to the state of Mississippi. Like, it would have been slightly unintuitive to have named it the Missouri River all the way down past those Gulf Coast states. The history of river names and deciding which name continues after a confluence is quite interesting.

It is an Algonquian name for the river, which the French transliterated when they explored the upper reaches.

The whole thing is called the Mississippi river because it was the first to be discovered and named, and the French name stuck (I forgot what the Spanish called it but that name didn’t stick)